Further work has been done on the 27 kD protein obtained from human plasma as a contaminant of TBG preparations. The protein is immunochemically distinct from TBG, but binds T4 with an association constant of 10 to the 7. It, therefore, appears to participate in T4 transport in blood. Studies on deglycosylation of TBG have shown that the oligosaccharide chains of TBG influence the structure of the apoprotein and also enhance its stability in acid and in guanidinium chloride solutions. T4 binding, in addition to enhancing the stability of TBG, prevents the structural change that accompanies deglycosylation. These findings are among the first to define an interaction between the carbohydrate and amino acid moieties of glycoproteins.